Health and Safety  
                                                                                                           Executive
             Volatile organic compounds 
             in air
             Laboratory method using sorbent tubes, solvent desorption 
             or thermal desorption and gas chromatography
                                         Scope
                                         1     Three methods are described for the determination of time-weighted average 
                                4        concentrations of volatile organic compounds (VOCs) using air sampling onto 
                                         sorbent tubes followed by gas chromatographic analysis. Method 1 uses pumped 
                                         sampling with sample introduction by thermal desorption, Method 2 uses diffusive 
                                         sampling also with sample introduction by thermal desorption and Method 3 uses 
                                0        pumped sampling and solvent desorption. All three methods are suitable for 
                                         personal and static sampling. 
                                         2     These methods are applicable to a wide range of VOCs. For Methods 1 and 
                                         2, two sorbents are recommended for general use for hydrocarbons, halogenated 
                                         hydrocarbons, esters, glycol ethers, ketones and alcohols. For Method 3 the most 
                                         common sorbent used is activated coconut shell charcoal although others are 
                                         available for specific applications.
                                         Summary
                                         3     A general guide to which of the three sorbent tube monitoring methods is 
                                         appropriate for volatile organic compounds dependent upon required sampling time 
                                         and estimated analyte concentration is given in Table 1.
                                         4     The upper limit of the useful range of the three methods is determined by the 
                                         sorptive capacity of the sorbent used and by the linear dynamic range of the gas 
                                         chromatograph column and detector. The lower limit of the useful range depends 
                                         on the noise level of the detector and on blank levels on the sorbent tubes or 
                                         desorption solvent.
                                         5     Method 1 (pumped sampling; thermal desorption) is suitable for sampling 
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                                         times of the order of a few minutes (0.1–100 mg.m ) up to 2 hours (low flow rate, 
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                                         approximate range 0.1–10 mg.m . A measured volume of air is drawn through a 
                                         sorbent tube containing appropriate sorbent, specifically selected for the compound 
                                         or mixture to be sampled. The collected vapour is then thermally desorbed in an 
                                MDHS1    inert carrier gas into a gas chromatograph fitted with a suitable capillary column 
                                         and detector. 
                                         6     Method 2 (diffusive sampling; thermal desorption) is suitable for samples of 
                                         greater than 15 minutes up to 8 hours over a concentration range of approximately 
                                                       -3
                                         1–1000 mg.m . The sorbent tube, fitted with a diffusion cap, is exposed for a 
                                         measured period of time, and then thermally desorbed in the same manner as 
               Methods for the           Method 1. 
               Determination of          7     Method 3 (pumped sampling; solvent desorption) is suitable for samples of a 
               Hazardous Substances      few minutes up to 8 hours over a concentration range of approximately 1–1000 
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               Health and Safety         mg.m . A measured volume of air is drawn through a sorbent tube, which is then 
               Laboratory                desorbed with a solvent, typically carbon disulphide for simple hydrocarbons.
                                                                                                           Health and Safety  
                                                                                                           Executive
                                          8    In all three methods, the desorbed samples are quantified by gas 
                                          chromatography (GC) using either a flame ionisation (FID), mass selective (MS), or 
                                          other suitable detector. 
                                          9    The use of alternative methods not included in the MDHS series is acceptable 
                                          provided they can demonstrate the accuracy and reliability appropriate to the 
                                          application. 
                                          Recommended sampling
                                          10  Method 1 (pumped sampling; thermal desorption): recommended sampling 
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                                          rate: 50–100 ml.min ; typical sampled volume: 1 to 10 litres; maximum sampled 
                                          volume: see Tables 2a, 2b and 3 for details corresponding to Tenax TA, Carbopack 
                                          X and Chromosorb 106 sorbents respectively. 
                                          11  Method 2 (diffusive sampling; thermal desorption); minimum sampling time:  
                                          15 – 30 minutes (up to 8 hours); see Table 4 for details of diffusive uptake rates for 
                                          a range of analyte/sorbent combinations.
                                          12  Method 3 (pumped sampling; solvent desorption); recommended sampling 
                                          rate: 50 – 200 ml.min-1; typical sample volume: 10 litres.
                                          Prerequisites
                                          13  Users of Methods 1 and 3 where pumped sampling is required will need to be 
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                                          familiar with the content of MDHS14.
                                          Safety
                                          14  Users of this method should be familiar with standard laboratory practice and 
                                          be able to carry out a suitable risk assessment. It is the user’s responsibility to 
                                          establish appropriate health and safety practices and to ensure compliance with 
                                          regulatory requirements.
                                          Equipment
                                          Sorbent tubes
                                          Methods 1 and 2
                                          15  Pre-packed stainless steel sorbent tubes. Typically, the tubes should be 
                                          packed with 200 mg to 300 mg of a suitable sorbent (or sorbents) of particle size 
                                          0.18 mm to 0.5 mm (35 to 80 mesh). The sorbent is usually retained between two 
                                          steel gauzes. Each tube should have metal storage end caps fitted with PTFE 
                                          seals. The inlet end of the tube is usually marked by a scored ring about 10 mm 
                                          from the end. Ideally the tube should have a unique engraved/etched identification 
                                          number or bar code. Avoid the use of adhesive labels on the body of the tube that 
                                          can interfere with the tube transfer mechanism in the thermal desorption 
                                          equipment.
                                          16  Diffusion end caps (Method 2 only). Standard end caps contain a metal gauze 
                                          only; special caps with an additional silicone membrane may be used to reduce 
                                          water uptake when using hydrophilic sorbents.
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                                          17  Select a sorbent tube appropriate for the compound or mixture and the 
                                          required sampling duration. Guidance on selection of suitable sorbent tubes is 
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                                          given in Tables 2a, 2b, 3 and in EN ISO 16017-1  for active sampling and in table 4 
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                                          and EN ISO 16017-2  for passive sampling. For active sampling, care should be 
                                          taken to avoid breakthrough of the analyte and the recommended safe sampling 
                                          volume (SSV) should be implemented. See Appendix 1 for further information on 
                                          breakthrough volume and SSVs. 
                                          18  The industry standard stainless steel sorbent tube dimensions are typically 
                                          either 3.5” (89 mm) long × ¼” (6.4 mm) OD × 5 mm ID or 7” (178 mm) long × 6 
                                          mm OD × 5 mm ID (not used passively, glass tubes with similar dimensions are 
                                          also available ). The air gap dimensions (distance between stainless steel screen 
                                          retaining the sorbent bed and the inlet end of the tube) which is fundamental for 
                                          accurate diffusive sampling should be in the range 14.0 mm to 14.6 mm. The 
                                          diffusive uptake rates given in Table 4 are based on the 3.5” stainless steel tube. 
                                          Additional information on the tube construction, dimensions, range of sorbent types 
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                                          and applicability are given in EN ISO 16017-1  or EN ISO 16017-2.
                                          19  The sorbent tubes should be conditioned according to the manufacturer’s 
                                          instructions before use, either on the instrument conditioning cycle or alternatively 
                                          under a flow of dry inert gas such as nitrogen at typically 100 ml min-1 for 10 
                                          minutes with gas flow in the opposite direction to that used for sampling. The tube 
                                          should be heated to a temperature usually at least 25 °C below the manufacturer’s 
                                          recommended maximum temperature for the sorbent but above the analysis 
                                          desorption temperature and recapped immediately after cooling. Guidance on 
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                                          typical conditioning parameters for common sorbents is given in EN ISO 16017-1  
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                                          or EN ISO 16017-2.  
                                          20  For new tubes, after conditioning, a representative sample, (typically 10%) 
                                          should be analysed to ensure that the thermal desorption blanks are acceptable. 
                                          The sorbent tube blank level should be very low (eg <2 ng) but with use of high 
                                          sensitivity detection methods very low sample amounts can be analysed. Rather 
                                          than stating absolute values, it may be considered acceptable if interfering artefact 
                                          peak areas are less than 5% of the typical area of the analyte(s). If levels of 
                                          artefacts exceed these levels, reconditioning should be considered. 
                                          21  The desorption efficiency of the analyte from the sorbent tube will be 
                                          quantitative (>95%) provided that appropriate desorption parameters are adopted. 
                                          Typical desorption temperatures are given in Tables 2a, 2b and 3 for a range of 
                                          organic compounds on Tenax TA, Carbopack X and Chromosorb 106 sorbents 
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                                          respectively. Information on additional sorbents can be found elsewhere.  The 
                                          efficiency of desorption can be checked by analysing a clean empty glass tube  
                                          (to identify that the analyte has been fully desorbed from the trap) after the sorbent 
                                          tube, followed by reanalysis of the first tube to confirm quantitative desorption has 
                                          been achieved by the first analysis. This should be repeated for a range of loadings 
                                          to cover the calibration range of the method. Alternative procedures to determine 
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                                          desorption efficiency are described elsewhere.
                                          Method 3
                                          22  A wide variety of pre-packed sorbent tubes for solvent desorption is 
                                          commercially available, with coconut shell charcoal amongst the most widely used. 
                                          Guidance on selection of suitable sorbent tubes is given in Table 5. Typical tubes 
                                          are made of glass 70 mm to 110 mm long with an OD of 6 mm to 8 mm. The 
                                          tubes are generally supplied with both ends flame sealed and usually containing 
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                                          two sorbent beds, separated and held in place by glass wool or foam plugs. In the 
                                          case of coconut shell charcoal, the front (sample) bed usually contains 100 mg of 
                                          sorbent and the back-up bed 50 mg. When sampling, glass tubes should be held 
                                          in a protective holder when sampling in order to prevent breakage.
                                          23  There may be some batch-to-batch variation in sorptive capacity of coconut 
                                          shell charcoal. Tubes from a single batch should ideally be used in each 
                                          application. Sorbents other than charcoal are available and may be used for certain 
                                          applications.4 
                                          24  The desorption efficiency (D) of sorbents may vary for type, batch and mass 
                                          loading. It is therefore necessary to determine D for each sorbent and analyte over 
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                                          the sample concentration range. See Appendix 2 or BS ISO 16200-1  for 
                                          procedures describing determination of D. 
                                          25  The sorbent tube sampling efficiency will be 100%, provided that the sampling 
                                          capacity of the sorbent has not been exceeded. If this capacity is exceeded, 
                                          breakthrough of vapour from the front section to the back-up section will occur. 
                                          The breakthrough volume varies with ambient air temperature, relative humidity, 
                                          concentration of sampled vapour and of other contaminants, and with the sampling 
                                          flow rate. An increase in any of these parameters causes a reduction in the 
                                          breakthrough volume (See Appendix 1 for further information on breakthrough 
                                          volume). The back-up section may be used as a check on breakthrough under 
                                          practical conditions. 
                                          26  Published methods and validation data giving detailed information on 
                                          breakthrough volumes, sampling flow rates, sampling times and analysis for 
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                                          specific analytes are available from the US regulatory authorities, ie OSHA  and 
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                                          NIOSH.
                                          Other equipment
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                                          27  Personal sampling pumps that meet the requirements of BS EN ISO 13137.
                                          28  A portable flow meter calibrated against a primary standard, with a 
                                          measurement uncertainty typically less than ±2%.
                                          29  Flexible plastic tubing for making a leak-proof connection from the sorbent 
                                          tube to the pump; belts or harnesses to facilitate attachment of sampling apparatus 
                                          to sample subjects.
                                          Laboratory reagents and apparatus 
                                          30  During analysis use only reagents of a recognised analytical quality.
                                          31  Volatile organic compounds: for preparation of calibration standards. 
                                          32  Methanol: diluent for the preparation of sorbent tube standards (Methods 1 
                                          and 2; alternative solvents can be used when appropriate).
                                          33  Carbon disulphide: for solvent desorption of sorbent tubes (Method 3) 
                                          34  A calibrated 5 µl syringe capable of dispensing accurate volumes between 1 
                                          µl and 5 µl: for preparation of spiked sorbent tube standards (Methods 1 and 2). 
                                          35  A range of volumetric flasks with caps or suba-seal closures, Class A 
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                                          complying with the requirements of BS EN ISO 1042.
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